High-k dielectric film, method of forming the same and related semiconductor device

ABSTRACT

A high-k dielectric film, a method of forming the high-k dielectric film, and a method of forming a related semiconductor device are provided. The high-k dielectric film includes a bottom layer of metal-silicon-oxynitride having a first nitrogen content and a first silicon content and a top layer of metal-silicon-oxynitride having a second nitrogen content and a second silicon content. The second nitrogen content is higher than the first nitrogen content and the second silicon content is higher than the first silicon content.

PRIORITY CLAIM

This application is a continuation of international applicationPCT/EP2003/050352, filed on Jul. 30, 2003, which is incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a high-k dielectric film, a method offorming the same and related semiconductor devices, and in particular tohigh-k dielectric films related to a gate dielectric for field effectsemiconductor devices or a capacitor dielectric for trench capacitors inintegrated circuits.

BACKGROUND

For forming semiconductor devices like CMOS devices (Complementary MetalOxide Semiconductor), MOSFET devices (Metal Oxide Semiconductor FieldEffect Transistor) or high memory devices such as DRAMs (Dynamic RandomAccess Memories), it is often useful to form a thin, high dielectricconstant (high-k) film onto a substrate, such as a silicon wafer. Avariety of techniques have been developed to form such thin films on asemiconductor wafer.

In the past, gate dielectric layers have been formed using silicondioxide. The scaling down of the above described devices, however, hasincreased the demand for gate dielectrics with a higher dielectricconstant than silicon dioxide. This is necessary to reach ultra thinoxide equivalent thicknesses (EOT, Equivalent Oxide Thickness) withoutcompromising gate leakage current.

In detail, as semiconductor devices have scaled to smaller dimensions,effective gate dielectric thickness has gotten thinner. The continuedscaling of conventional gate dielectrics, such as SiO₂ and SiO_(x)N_(y),has almost reached the fundamental limits of very high gate leakagecurrent, due to direct tunneling, which is not acceptable in a scaleddevice requirement of a low leakage current. In order to suppress thehigh leakage current, several high-k films of transition metal oxide andsilicate, such as HfO₂, ZrO₂, Hf-aluminate, Zr-aluminate, Zr-silicate,Hf-silicate and a lanthanide oxide like La₂O₃, Pr₂O₃, and Gd₂O₃, havebeen studied in replacement of SiO₂ and SiO_(x)N_(y).

However, these conventional materials have shown a number ofdisadvantages. According to S. OHMI, et al., “Rare earth metal oxidegate thin films prepared by E-beam deposition”, International Workshopon Gate Insulator 2001, Tokyo, Japan, it is known that ZrO₂ or HfO₂ hasshown micro crystal formation, resulting in high leakage current.

Furthermore, from J. H. LEE, et al., “Poly-Si gate CMOSFETs withHfO₂—Al₂O₃ laminate gate dielectric for low power applications”, Tech.Dig. VLSI, page 84, 2002, it is known that HfO₂—Al₂O₃ laminate orHf-aluminate have serious mobility degradation due to fixed charges inthe high-k dielectric film.

Moreover, TAKESHI, YAMAGUCHI, et al., “Additional scattering effects formobility degradation in Hf-silicate gate MISFETs” Tech. Dig. IEDM 2002reports that in case of Zr-silicate or Hf-silicate, phase separation ofthe film into HfO₂ and SiO₂ regions by the high temperature annealinduces also mobility degradation.

For lanthanide oxides, leakage current results have indicated thatlanthanide oxides may be possible candidates of future dielectrics.However, according to H. IWAI, et al., “Advanced gate dielectricmaterials for Sub-100 nm CMOS”, Tech. Dig. IEDM 2002, it is reportedthat these lanthanide oxides also form interfacial layers on Sisubstrates after subsequent thermal annealing, which may indicatethermal instability of these lanthanide oxides.

Moreover, impurity penetration such as boron penetration from e.g. agate layer to a Si substrate is a further problem to be solved by thesehigh-k dielectric films. Even though nitrogen incorporation onHfSi_(x)O_(y) has been known to suppress impurity penetration (e.g.boron penetration) and improve thermal stability, it was also reportedthat a very high Si content of Si/[Si+Hf] ratio of over 80% inHfSi_(x)O_(y)N_(z) prevents flat band voltage shift, resulting inseriously reducing dielectric constant of the film even with a highnitrogen content of 30 atomic percent (see M. KOYAMA, et al. “Effects ofnitrogen in HfSiON gate dielectric on the electrical and thermalcharacteristics”, Tech. Dig. IEDM 2002, 34-1). This high Si contentlow-k dielectric film of HfSi_(x)O_(y)N_(z) is almost the same asconventional SiO_(x)N_(y) in terms of dielectric constant. Othertechnical problems of the formation of respective films on Si substrateare that a high nitrogen concentration at the interface between thedielectric layer and the Si substrate can be induced by subsequent highthermal annealing to degrade mobility.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not limitedto the accompanying figures in which like references indicate similarelements. Exemplary embodiments will be explained in the following textwith reference to the attached drawings, in which:

FIGS. 1A and 1B are partial cross-sectional views showing production ofa high-k dielectric film according to a first embodiment;

FIG. 2 is a partial cross-sectional view of the high-k dielectric filmaccording to a second embodiment;

FIGS. 3A to 3D are partial cross-sectional views showing production of ahigh-k dielectric film according to a third embodiment;

FIG. 4 is a partial cross-sectional view of a field effect semiconductordevice using the high-k dielectric film; and

FIG. 5 is a partial cross-sectional view of a trench capacitor using thehigh-k dielectric film.

Skilled artisans appreciate that elements in the figures are illustratedfor simplicity and clarity and have not necessarily been drawn to scale.

DETAILED DESCRIPTION

FIGS. 1A and 1B show partial cross-sections illustrating production of ahigh-k dielectric film according to a first embodiment. FIG. 1Aillustrates a semiconductor substrate, such a monocrystalline Sisubstrate 1, is used as a base. On the surface of the substrate 1, abottom layer B of metal-silicon-oxynitride (MSi_(xB)O_(yB)N_(zB)) isformed by layer deposition methods.

As shown in FIG. 1B, a top layer T is formed by the samemetal-silicon-oxynitride on the surface of the bottom layer B. However,the top layer T has different silicon content and nitrogen content thanthe bottom layer B. In more detail, the deposited top layer T containsMSi_(xT)O_(yT)N_(zT) having a second nitrogen content zT and a secondsilicon content xT. The nitrogen content zB and zT of the bottom and toplayer B and T as well as the silicon content xB and xT are selected suchthat the nitrogen content zT of the top layer T is higher than thenitrogen content zB of the bottom layer and the silicon content xT ofthe top layer T is higher than the silicon content xB of the bottomlayer.

The indices x, y and z or xB, yB and zB or xT, yT and zT in theMSi_(x)O_(y)N_(z) layers are real positive values indicating molecularatomic percentage. Nitrogen atomic percentage in respective layer iscalculated by (z/(1+x+y+z))×100. Usually, Si and M (metal)concentrations are presented by (x/(1+x))×100 and (1/(1+x))×100,respectively.

The bilayer stack shown in FIG. 1B constitutes a high-k dielectric film2. In more detail, the high-nitrogen-content top layer T of the bilayerstack prevents diffusion of e.g. boron from a boron-doped poly-Sielectrode (not shown) into the field effect device channel, whereas thelow-nitrogen-content bottom layer B is decreases mobility degradationwithin this channel and/or the substrate 1. Moreover, since a decreaseof Si content in metal-silicon-oxynitrides is related to an increase ofthe metal content and, in turn, an increase of the dielectric constant,the bilayer stack provides a higher dielectric constant than a similarhigh-Si-content metal-silicon-oxynitride. Thus, it is possible tofurther scale down an effective oxide thickness (EOT), while leakagecaused by direct tunneling is reduced.

Si and N concentrations in the top layer T are higher than those in thebottom layer B, e.g. xT>xB, zT>zB. A ratio of N and Si in the top layerT with respect to the bottom layer B depends on whatmetal-silicon-oxides are used. If Hf is used as the metal M in themetal-silicon-oxynitride layer (HfSi_(x)O_(y)N_(z)), the Si/(Hf+Si)ratio of HfSi_(x)O_(y)N_(z) is in the range of about 70% to 95% with anN concentration of as high as about 30 atomic percent. Thus, for a fixedSi and N concentration in the top layer T using Hf as metal, the N andSi content of the bottom layer B are lower than those of the top layerT, with the N atomic percentage in the range from about 15% to 25% and aSi/(Hf+Si) ratio in the range from about 20% to 60%.

Concerning the thickness of the different layers, the top layer T has asmaller thickness than the thickness of the sum of the other layerswithin the high-k dielectric film 2 or the same. That is, the top layerT in the bilayer structure of the first embodiment is thinner or equalto the bottom layer B.

After formation of the high-k dielectric film 2, an anneal process maybe used to increase the density of the layer stack and to reduce defectsto improve the quality of the high-k dielectric film 2 such as toimprove the leakage current characteristics. The anneal process mayoccur at a temperature of over 600° C. in N₂ or other gas.

FIG. 2 shows a partial cross-sectional view of a high-k dielectric filmaccording to a second embodiment. The same reference numbers refer tothe same or corresponding layers and a repeated description of theselayers is omitted. According to the second embodiment, a substrateinterface layer 1 is formed on the surface of the substrate 1. Therespective top and bottom layers T and B of the high-k dielectric film 2are formed on the surface of the substrate interface layer I. If a Sisubstrate 1 is used, the substrate interface layer may contain a siliconoxide, which further increases the mobility within the substrate andreduces defects at the surface of the substrate 1.

Besides the bilayer MSi_(x)O_(y)N_(z) stack of the first and secondembodiment having a bottom layer and a top layer ofmetal-silicon-oxynitride, at least another metal layer ofmetal-silicon-oxynitride may be formed between the bottom layer B andthe top layer T. This additional layer(s) may have nitrogen and siliconcontents between the nitrogen and silicon contents of the bottom layer Band top layer T. In particular, the nitrogen and silicon content of thismiddle layer is higher than the nitrogen and silicon contents of ametal-silicon-oxynitride layer formed in a layer formed previously. Thethickness of the top layer T is again equal to or less than the sum ofthicknesses of the remaining layers in the high-k dielectric film ormultilayer stack, i.e. the bottom layer and the middle layer(s).

Physical vapor deposition (PVD) may be used to deposit variousMSi_(x)O_(y)N_(z) layers. In one embodiment, co-sputtering of metal,such as Hf, Zr, La, Pr, Gd and other lanthanide metals, and silicon inan Ar/N₂/O₂ ambient may be used to form metal-silicon-oxynitride films.Nitrogen concentration and silicon concentration in thesemetal-silicon-oxynitrides can be controlled by N₂ flow and siliconsputtering rate. High nitrogen content metal-silicon-oxynitride with asmall amount of silicon can be obtained using high-nitrogen flow duringthe co-sputtering of Si. A poly-Si electrode (not shown) mayadditionally be formed on the top of the high-k dielectric film in anyembodiment.

In one embodiment, a high nitrogen content and high silicon contentmetal-silicon-oxynitride at the top part of the high-k dielectric film 2and a lower nitrogen content and a lower silicon content at the bottompart of the high-k dielectric film 2 can be formed to keep the interfaceof the high-k dielectric film 2 and the poly-Si electrode, if formed,thermally stable. Poly-Si electrode deposition usually is performedusing SiH₄ or Si₂H₆. This may induce reduction of metal-O and metal-Nbonds by hydrogen coming from SiH₄ or Si₂H₆. The reduction may generatedefects and, in turn, result in high leakage current. High nitrogenconcentrations and high silicon concentrations inmetal-silicon-oxynitride suppress reaction between hydrogen and thehigh-k dielectric film during a poly-Si deposition due to a number ofSi—N and Si—O bonds in the place of metal-O and metal-N bonds. Similaradvantages result when using a metallic material instead of poly-Si asan electrode formed on the surface of the high-k dielectric film 2.

Thus, ZrSi_(x)O_(y)N_(z), HfSi_(x)O_(y)N_(z), LaSi_(x)O_(y)N_(z),PrSi_(x)O_(y)N_(z), GdSi_(x)O_(y)N_(z), DySi_(x)O_(y)N_(z), and othernitrogen incorporated lanthanide-silicates can be formed as high-kdielectric films.

FIGS. 3A to 3D show partial cross-sectional views illustrating steps forproducing a high-k dielectric film according to a third embodiment.Again, the same reference numbers refer to same or corresponding layersas in FIGS. 1 and 2, and, therefore, a repeated description of theselayers is omitted in the following. According to the third embodiment atriple layer stack is formed by an alternative method to provide thehigh-k dielectric film 2.

According to FIG. 3A, a substrate interface layer I is formed directlyon the surface of the substrate 1, for example by a thermal process.Thus, a SiO₂-substrate interface layer I is formed on the Si substrate1. A pre-bottom layer B1 of metal-silicon-oxide having a low firstsilicon content xB is then formed by a method such as deposition on thesubstrate interface layer I.

According to FIG. 3B, a pre-middle layer M1 of metal-silicon-oxidehaving a silicon content xM is deposited on the pre-bottom layer B1having the silicon content xB. The silicon content xM of the layer orthe pre-middle layer M1 is higher than that of the bottom layer B1 or(if a plurality of middle layers are to be used) is higher than that ofthe preceding layer.

According to FIG. 3C, a pre-top layer T1 of metal-silicon-oxide having asilicon content xT is formed on the pre-middle layer M1. The secondsilicon content xT of the pre-top layer T1 is higher than the that ofthe pre-middle layer M1 which is higher than that of the pre-bottomlayer B1.

Furthermore, an N₂ plasma treatment is performed on themetal-silicon-oxide layers forming SiN bond rather than metal-N bond.Thus, as the Si content in the different layers is different, thenitrogen content after the N₂ plasma treatment is different. In oneexample, it is known that HfSi_(x)O_(y) can be formed by variousdeposition methods, such as MOCVD (Metal Organic Chemical VaporDeposition), PVD (Physical Vapor Deposition), or ALD (Atomic LayerDeposition). Using MOCVD HfSi_(x)O_(y), two precursors, Hf[N(C₂H₅)₂]₄for Hf and Si[N(CH₃)₂]₄ for Si, flow into a reactor together with 02 todeposit HfSi_(x)O_(y). The SiO₂ mole fraction in HfSi_(x)O_(y) can becontrolled by controlling the process parameters, such as temperature,pressure and both precursor flow rates, during the silicate deposition.

Increasing the temperature from 325° C. to 650° C. increases the SiO₂mole fraction in HfSi_(x)O_(y) from about 20% to 65%. Changing theprocess pressure from about 400 Pa (3 Torr) to 1065 Pa (8 Torr) resultsin increasing the SiO₂ mole fraction from about 30% to 45%.

In the third embodiment, MOCVD HfSi_(x)O_(y) is used. A sequence fordepositing low SiO₂ content HfSi_(xB)O_(yB) and high SiO₂ contentHfSi_(xT)O_(yT) are deposited at the bottom part and at the top part ofthe high-k dielectric film 2 and then followed by N₂ plasma treatment.After N₂ plasma nitration, the top layer T of the film 2 has a highnitrogen content and the bottom layer B has a low nitrogen content.

Alternatively, a Zr silicate can be used instead of MOCVD Hf silicate.In this case, the precursors used to form the Zr silicate using MOCVDinclude Zr[N(C₂H₅)₂]₄ and Si[N(CH₃)₂]₄. In other embodiments, MOCVDlanthanide silicates can be formed using similar lanthanide MOCVDprocesses.

Thus, according to FIG. 3D, after performing a N₂ plasma treatment, thepre-bottom layer B1, the pre-middle layer M1 and the pre-top layer T1are converted to a respective metal-silicon-oxynitride bottom layer B,middle layer M and top layer T. An annealing process may again beperformed after the N₂ plasma treatment to repair the plasma damage tothe high-k dielectric film 2, to increase the density of the high-kdielectric film, and/or to reduce defects as well as impurities in thehigh-k film stack.

FIG. 4 shows a partial cross-sectional view of a semiconductor devicecomprising a field effect transistor using the high-k dielectric film asa gate dielectric. In FIG. 4, a source region S, a drain region D and achannel region between the source and drain region are provided in asemiconductor substrate 1. The high-k dielectric film 2 is used as gatedielectric and formed on the channel region while a gate layer is formedon the gate dielectric 2. A spacer SP may be provided at the side wallsof the gate stack. The spacer SP may aid in the formation of the sourceand drain regions S and D. The gate layer 3 may comprise a poly-Si or ametal gate. Use of a metal gate may improve the electricalcharacteristics of the semiconductor device.

FIG. 5 shows a partial cross-sectional view of a semiconductor device inwhich the high-k dielectric film is used as a capacitor dielectric.Trench capacitors may be used in different semiconductor devices suchas, e.g. DRAM (Dynamic Random Access Memory) devices. As shown in FIG.5, a deep trench or hole is provided within the silicon substrate 1. Thehigh-k dielectric film 2 is provided on the surface of the trench orhole after forming a second electrode 5 in the substrate 1 in thevicinity of the lower part of the trench. A filling material 4 is usedas a first electrode of the resulting capacitor. The filling material 4is usually highly doped poly-Si or a deposited metal. Thus, the high-kdielectric film improves the electric characteristics of semiconductordevices thereby enabling increased integration densities.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that a high-k dielectric film is provided. In addition,a method is provided for forming the high-k dielectric film as well asrelated semiconductor devices having an improved leakage current byreduced tunneling currents as well as improved diffusion barriercharacteristics. Further modifications and alternative embodiments ofvarious aspects of the invention will be apparent to those skilled inthe art in view of this description.

It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, that areintended to define the spirit and scope of this invention. Nor isanything in the foregoing description intended to disavow scope of theinvention as claimed or any equivalents thereof.

1. A high-k dielectric film comprising: a metal-silicon-oxynitridebottom layer having a first nitrogen content and a first siliconcontent; and a metal-silicon-oxynitride top layer on the bottom layer,the top layer having a second nitrogen content and a second siliconcontent, the second nitrogen content higher than the first nitrogencontent and the second silicon content higher than the first siliconcontent.
 2. The high-k dielectric film of claim 1, further comprising asilicon oxide substrate interface layer formed between the bottom layerand a substrate on which the high-k dielectric film is disposed.
 3. Thehigh-k dielectric film of claim 1, further comprising ametal-silicon-oxynitride middle layer between the bottom layer and thetop layer, the middle layer having a third nitrogen content between thefirst and second nitrogen content and a third silicon content betweenthe first and second silicon content.
 4. The high-k dielectric film ofclaim 3, wherein at least two of the bottom, middle, or top layerscomprise the same metal-silicon-oxynitride.
 5. The high-k dielectricfilm of claim 1, wherein the metal-silicon-oxynitride of at least one ofthe bottom and top layers comprises at least one of ZrSi_(x)O_(y)N_(z),HfSi_(x)O_(y)N_(z), LaSi_(x)O_(y)N_(z), PrSi_(x)O_(y)N_(z),GdSi_(x)O_(y)N_(z) or DySi_(x)O_(y)N_(z), wherein x, y and z are realpositive values indicating molecular atomic percentage.
 6. The high-kdielectric film of claim 1 wherein a thickness of the top layer is atmost the sum of thicknesses of the remaining layers in the high-kdielectric film.
 7. A semiconductor device comprising a substrate and ahigh-k dielectric film on the substrate, the high-k dielectric filmcomprising: a metal-silicon-oxynitride bottom layer having a firstnitrogen content and a first silicon content; and ametal-silicon-oxynitride top layer on the bottom layer, the top layerhaving a second nitrogen content and a second silicon content, thesecond nitrogen content higher than the first nitrogen content and thesecond silicon content higher than the first silicon content.
 8. Thesemiconductor device of claim 7, wherein the substrate is asemiconductor substrate, the semiconductor device comprises a fieldeffect transistor that includes a source region, a drain region, and achannel region provided in the semiconductor substrate, the high-kdielectric film forming a gate dielectric on the channel region, and agate layer formed on the gate dielectric.
 9. The semiconductor device ofclaim 7, wherein the substrate is a semiconductor substrate, thesemiconductor device comprises a trench capacitor with a firstelectrode, a capacitor dielectric, and a second electrode formed in thesemiconductor substrate, the capacitor dielectric containing the high-kdielectric film.
 10. The semiconductor device of claim 7, wherein thehigh-k dielectric film further comprises a silicon oxide substrateinterface layer formed between the bottom layer and the substrate. 11.The semiconductor device of claim 7, wherein the high-k dielectric filmfurther comprises a metal-silicon-oxynitride middle layer between thebottom layer and the top layer, the middle layer having a third nitrogencontent between the first and second nitrogen content and a thirdsilicon content between the first and second silicon content.
 12. Amethod of forming a high-k dielectric film, the method comprising: a)forming a metal-silicon-oxynitride bottom layer on a substrate, thebottom layer having a first nitrogen content and a first siliconcontent; b) forming a metal-silicon-oxynitride top layer on the bottomlayer, the top layer having a second nitrogen content and a secondsilicon content, the second nitrogen content higher than the firstnitrogen content and the second silicon content higher than the firstsilicon content.
 13. The method of claim 12, further comprising forminga silicon oxide substrate interface layer on the substrate beforeforming the bottom layer and forming the bottom layer on the interfacelayer.
 14. The method of claim 12, further comprising forming ametal-silicon-oxynitride middle layer between the top and bottom layers,the middle layer having a third nitrogen content higher than the firstnitrogen content and a third silicon content higher than the firstsilicon content.
 15. The method of claim 14, wherein at least one of thebottom, middle or top layers are formed by co-sputtering of metal andsilicon in Ar/N₂/O₂ ambient, a nitrogen and silicon concentrationcontrolled by N₂ flow and Si sputtering rate.
 16. A method of forming ahigh-k dielectric film, the method comprising: a) forming ametal-silicon-oxide pre-bottom layer on a substrate, the pre-bottomlayer having a first silicon content; b) forming a metal-silicon-oxidepre-top layer on the pre-bottom layer, the pre-top layer having a secondsilicon content higher than the first silicon content; and c) performingN₂ plasma treatment to convert the pre-bottom layer to ametal-silicon-oxynitride bottom layer having a first nitrogen contentand the pre-top layer to a metal-silicon-oxynitride top layer having asecond nitrogen content higher than the first nitrogen content.
 17. Themethod of claim 16, further comprising forming a metal-silicon-oxidepre-middle layer between the pre-top layer and the pre-bottom layer, thepre-middle layer having a third silicon content higher than that of thepre-bottom layer, wherein, during the N₂ plasma treatment, thepre-middle layer is converted to a metal-silicon-oxynitride middle layerhaving a third nitrogen content higher than that of the pre-bottomlayer.
 18. The method of claim 17, wherein at least one of thepre-bottom, pre-middle, or pre-top layers are formed by at least one ofMOCVD, PVD, or ALD.
 19. The method of claim 16, further comprisingannealing at least one of the bottom, middle, or top layers.
 20. Themethod of claim 16, further comprising forming a silicon oxide substrateinterface layer on the substrate before forming the pre-bottom layer,wherein the pre-bottom layer is formed on the interface layer.
 21. Themethod of claim 16, wherein the metal-silicon-oxynitride of at least oneof the bottom and top layers comprises at least one of:ZrSi_(x)O_(y)N_(z), HfSi_(x)O_(y)N_(z), LaSi_(x)O_(y)N_(z),PrSi_(x)O_(y)N_(z, GdSi) _(x)O_(y)N_(z), or DySi_(x)O_(y)N_(z), whereinx, y and z are real positive values indicating molecular atomicpercentage.
 22. A semiconductor device comprising a substrate,electrically conductive regions at least one of in or on the substrate,and means for reducing tunneling currents associated with currentspassing between the electrically conductive regions.
 23. Thesemiconductor device of claim 22, wherein the substrate is asemiconductor substrate, and the semiconductor device comprises a fieldeffect transistor that includes a source region, a drain region, and achannel region provided in the semiconductor substrate, and a gate layerformed on the means for reducing tunneling currents above the channelregion, the electrically conductive regions including the source anddrain regions.
 24. The semiconductor device of claim 22, wherein thesubstrate is a semiconductor substrate, and the semiconductor devicecomprises a trench capacitor with a first electrode, the means forreducing tunneling currents, and a second electrode formed in thesemiconductor substrate, the electrically conductive regions includingthe first and second electrodes.
 25. The semiconductor device of claim22, wherein the means for reducing tunneling currents comprises multiplemetal-silicon-oxynitride layers of silicon and nitrogen contents thatincrease with distance from the substrate.
 26. The semiconductor deviceof claim 25, wherein the means for reducing tunneling currents furthercomprises an interface layer formed between a bottommostmetal-silicon-oxynitride layer and the substrate, the interface layerhaving a different composition than the bottommost layer.